1,1,1,2,2-pentafluoroethane (HFC-125), which is a useful hydrofluorocarbon compound for its zero ozone depleting potential, is used in various applications such as foaming agent, propellant, refrigerant and the like.
Known processes for preparing HFC-125 include a process comprising fluorinating 2-chrolo-1,1,1,2-tetrafluoroethane (HCFC-124) with hydrogen fluoride (HF) in the gas phase. According to this process, in addition to the desired HFC-125, HC1 is produced as a by-product which reacts with HCFC-124 (reverse reaction) to produce 2,2-dichloro-1,1,1-trifluoroethan (HCFC-123). Further, the process produces impurities including chrolofluoroethanes (CFCs) such as 1,2,2-trichrolo-1,1,2-trifluoroethane (CFC-113), 2,2,2-trichrolo-1,1,1-trifluoroethane (CFC-113a), 1,2-dichrolo-1,1,2,2-tetrafluoroethane (CFC-114), 2,2-dichrolo-1,1,1,2-tetrafluoroethane (CFC-114a), 2-chrolo-1,1,1,2,2-pentafluoroethane (CFC-115), etc.; 2-chrolo-1,1,1-trifluoroethane (HCFC-133a), 1,1,1,2-tetrafluoroethane (HFC-134a) and the like.
The unreacted HCFC-124 and HCFC-123 formed in the reverse reaction can be converted into the desired HFC-125 by fluorination and, therefore, they can be recycled to the process and economically reused as starting materials. On the contrary, the CFCs, HCFC-133a and HFC-134a can not be fluorinated to form HFC-125 and, therefore, the ratios of these components in the reaction product need to be minimized to improve the yield of HFC-125. Especially, CFC-115 is difficult to be separated from HFC-125 because it has a boiling point close to that of HFC-125, which leads to a degraded purity of the desired HFC-125. Therefore, production of CFC-115 should be kept as low as possible. Further, CFC-113a and CFC114a have close boiling points to those of HCFC-123 and HCFC-124, which necessitates a fractionator having a large number of trays in order to separate them completely. If HCFC-123 and HCFC-124 are recycled to the process as starting materials without being purified by a fractionator, CFC-113a and CFC-114a are unavoidably contained in the starting materials. These CFCs are fluorinated in the process to eventually give CFC-115, thereby further degrading the purity of CFC-125.
As described above, the separation of CFC-115 from CFC-125 is difficult, and the separation necessitates additional apparatuses for extractive distillation and the like, which leads to increased costs of equipment and production. Besides, the ban on CFCs has been agreed by international conventions and, therefore, the production should be decreased. Thus, there is a demand for a process for the production of HFC-125 by fluorinating HCFC-124 wherein the formation of CFCs is suppressed as low as possible.
Among the known processes for the preparation of HFC-125 using HCFC-124 as a starting material with reduced production of CFCs, U.S. Pat. No. 5,475,167 discloses a process which uses as catalyst Cr.sub.2 O.sub.3 having a high surface area or Cr.sub.2 O.sub.3 pretreated with CO, H.sub.2 or H.sub.2 O. However, the process requires maintaining the production ratio of HFC-125 at a level of 50% or higher. According to the Examples in this U.S. patent, it is necessary to conduct the reaction at a high temperature of 350.degree. C. or higher with relatively long contact time of 6.8 (g.multidot.s/cc) for achieving the above production ratio. Further, it is observed from the Examples of the above patent that the production of CFCs is not suppressed satisfactorily. Specifically, though the production of CFCs is lowest in the case where a high surface area Cr.sub.2 O.sub.3 catalyst is used, the production ratio of CFCs (ratio of CFCs relative to HFC-125 produced) therein is about 3000 ppm. In other cases, the production ratios are 5000 ppm or larger.
U.S. Pat. No. 5,334,787 discloses a process for preparing HFC-125 by reacting in the gas phase HCFC-123 or HCFC-124 used as a starting material with HF in the presence of a Cr.sub.2 O.sub.3 catalyst. However, it is necessary also in this process to maintain the production ratio of HFC-125 at 50% or higher for inhibiting the production of CFCs, which results in the relatively long contact time of 10-100 seconds. The production of the CFCs is not satisfactorily suppressed also by the process.
Furthermore, U.S. Pat. No. 5,399,549 discloses a process for preparing HFC-125 by reacting in the gas phase HCFC-123 or HCFC-124 used as a starting material with HF in the presence of a Cr.sub.2 O.sub.3 catalyst. However, also in this process, the production of CFCs is not satisfactorily suppressed.